by Catalina Camia, USA TODAY

by Catalina Camia, USA TODAY

Democratic Sens. Heidi Heitkamp of North Dakota and Joe Donnelly of Indiana said Friday they now support gay marriage, joining more than a half-dozen of their colleagues in recent weeks to change their positions and endorse the controversial issue.

Their support - along with Sen. Bill Nelson, D-Fla., who came on board Thursday - means four Democrats have not endorsed same-sex marriage. They are Sens. Mark Pryor of Arkansas and Mary Landrieu of Louisiana, who face potentially competitive re-election bids in 2014; Tim Johnson of South Dakota, who is retiring; and Sen. Joe Manchin of West Virginia.

"I have concluded the federal government should no longer discriminate against people who want to make lifelong, loving commitments to each other or interfere in personal, private, and intimate relationships," Heitkamp said in a statement. "I view the ability to marry as a logical extension of this belief. The makeup of families is changing, but the importance of family is enduring."

There has been a steady stream of Democrats - but only two Republicans, Sens. Rob Portman of Ohio and Mark Kirk of Illinois - to reverse course recently and declare their support for gay marriage. Much of the action has come as the Supreme Court has heard arguments last week in two cases challenging the constitutionality of gay marriage.

Donnelly acknowledged the cases before the high court caused him to rethink his past positions and votes as a House member. "In doing so, I have concluded that the right thing to do is to support marriage equality for all," he said in a Facebook post.

The justices sounded skeptical about upholding the federal Defense of Marriage Act, which denies benefits to gay couples who are married. So timing is everything when assessing these political statements. Sen. Kay Hagan, D-N.C., so far is the only Democrat facing voters next year to change her position and support gay marriage. Donnelly and Heitkamp were elected to the Senate in November, and don't face voters again until 2018.